Process for inhibiting the deterioration of coating composition overspray



United States Patent PROCESS FOR INHIBITING THE DETERIORATION OF COATING COMPOSITION OVERSPRAY John E. Shamp, Perrysburg, Ohio, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application June 5, 1951, Serial No. 230,071

5 Claims. (Cl. 260-43) This invention relates to improved processes for recovering sprayed organic coating compositions. More particularly, this invention relates to a method of mhibiting the deterioration of accumulated quantities of the spray dust or overspray of sprayed organic coat1 ng compositions, which deterioration occurs during the mterval between spraying and the final steps of reprocessing the collected material into a usable coating composition.

In the spray application of paint, varnish, lacquer and the like some of the atomized coating composition is not deposited on the article being coated. This socalled spray dust or overspray is carried away by air in the spraying system and in many instances wasted. Since the overspray contains valuable organic hinder or film-former (such as vegetable oils, synthetic resins, cellulose derivatives, plasticizers, and the like) along with some solvents and/or diluents, and pigment if such is present in the product being sprayed; it is customary for large users of sprayed coatings to collect the overspray for subsequent reprocessing into usable coating compositions. Overspray is usually collected by passing the discharged air from a spraying operation through a water curtain and/ or water spray system which entraps or precipitates the sprayed particles and carries them to a collecting reservoir. The water usually contains an additive which aids in causing the particles to coagulate or agglomerate into a layer which is relatively easily separable from the bulk of the aqueous precipitating medium. This layer which is called sludge is removed from time to time and is usually packaged in drums for transportation to the recovery plant. The sludge contains the above-mentioned valuable constituents and a large proportion of undesirable occluded water or aqueous precipitating medium.

The conventional reclamation processes involve removing substantially all of the Water or aqueous precipitating medium by decantation, centrifugation, azeotropic distillation, and the like. In some cases the water-containing sludge is converted into an oil-in-water type of emulsion paint by adding an emulsifying agent, solvent or diluent, and more water if necessary. However, whatever the method of reclamation, if the overspray is to be converted into a usable coating composition having good protective or decorative properties as compared to the coating composition from which it was derived, the reclamation process must be carried out while the collected overspray is still completely soluble in a solvent of the same general character as the solvent in the original coating composition. For reasons which have not been completely investigated and established, overspray sludge deteriorates ultimately to a condition in which it is impossible or prohibitively expensive to reclaim, or from which it is impossible or prohibitively expensive to obtain a reclaimed product having protective or decorative properties substantially equal to those of the coating composition from which it Was derived.

The rate of this deterioration appears to depend 1n part upon the nature and constituents of the sprayed coating composition, the conditions under which it was sprayed and the overspray was collected, the time elapsed between spraying and reclamation, and the storage conditions during that period. Prolonged exposure to high temperatures, as would be encountered in storing the sludge-containing drums in the summer sun in a recovery plant yard, appears to promote deterioration faster than any other factor. The usable overspray portion of 'ice the sludge, even under the best of the above-mentioned conditions, progressively and gradually deteriorates from its original state, in which it is still soluble in its original kind of solvent, to a thickened state containing discrete insoluble particles commonly called seed or to a soft gel having limited solubility, and finally to a semi-solid gel having substantially no solubility in the kind of solvent used in the original composition. Once the stored sludge has deteriorated to the seedy or soft gel stage, or further to the semi-solid gel stage, it cannot be economically reconditioned and must be disposed of as waste or as a coating composition of markedly inferior quality in which poor durability or poor appearance are not objectionable.

An example of reprocessing to obtain a coating composition having protective or decorative properties substantially equal to the original coating composition is the production of a white synthetic enamel top coat for steel cabinets from the undeteriorated overspray of a new white synthetic enamel for the same purpose. The reprocessed material is usable alone as a high quality product, or it may be blended in any proportion with new enamel having substantially the same composition.

An example of the partial loss involved in reprocessing overspray sludge to obtain a coating composition having slightly inferior properties is the production, from the partially deteriorated overspray of a first quality white synthetic enamel top coat, of a second-quality white synthetic enamel top coat for cheaper steel cabinets on which the finish is not expected to yield the durability or appearance of a top quality product.

An example of the even greater loss involved in reprocessing overspray sludge to obtain a markedly inferior coating composition is the production, from the gelled overspray of a first-quality white synthetic enamel top coat, of a third-quality cheap general purpose paint which is useful only for the short term protection or decoration of such surfaces as temporary industrial buildings, fences, structures and equipment requiring continual repainting, and the like.

Thus the processors of overspray sludge have the everpresent alternate of reprocessing the sludge quickly to use it while it is in good condition or to take the losses involved in permitting the sludge to deteriorate.

The principal object of this invention is to provide a reclamation process for coating composition overspray sludge whereby the valuable constituents thereof can be reprocessed after prolonged storage under adverse conditions into a coating composition having properties substantially equal and not markedly inferior to the coating composition from which the overspray was derived.

Another object of this invention is to provide a process for treating collected overspray sludge to keep it in its original soluble state, whereby it can be reprocessed into a coating composition having susbtantially the protective and decorative properties of the original composition from which the overspray was derived and whereby the reprocessing can be done at a time substantially later than when the same sludge, without treatment, would have deteriorated to an unusable condition for the same purpose.

A further object is to provide an inhibitory or presewative treatment for coating composition overspray sludge whereby storage without excessive deterioration can be lengthened materially and whereby the scheduling of reprocessing can be made less dependent on the age of the sludge.

A still further object is to eliminate the loss of valuable high quality composition constituents which would otherwise be destroyed or used inefliciently in recovered coating compositions of very much lower quality.

These objects are accomplished by preserving and reprocessing overspray sludge by a combination of interdependent process steps consisting essentially of (l) placing in a closable storage vessel a water-bearing sludge containing the recovered overspray of an organic coating composition, the organic film-forming portion of which is completely soluble in a volatile organic solvent of the same character as the volatile organic solvent portion of the coating composition from which the overspray was derived, (2) adding to the sludge a volatile organic solvent for the film-forming portion of the overspray in sufii- *3 cient amount to substantially prevent the formation of seed or geltherein ufi'tilthesludg has been completely reprocessed, the film-forming portion being susceptible to the formation of seed orgelin the absence of the added solvent,"(3) 'subsequentlyreclaiming the 'overspra ".the 'sludge by separatingrtherefrom substantially "water and by dissolvin g the .overspray in' awolatile or- 1 game solvent of 'the 'same"chara'cter "as i: that contained-in i athefco'atin g composition from iwhich the overspray Was "-derived. FBy thisrcombirration of process steps'is'bbtained a reclaimed coating composition: having'protective and decorative properties substantially equal to those of "ithe'-'original coating "compositiorrfrom' which the overspray'was derived.

lThePfollowingf examples "illu'stratefbut do not limit the invention. I

EXAMPLE I white shaking-type: syntheticnrenamel :top coat for refrigerator cabinets-was. useduin. this. example. It con- I--sisted :essentially. of titanium-dioxide pigment uniformly aground. or dispersed-in a'. resinous: .filrmforrningv material, :the.essential constituents .of which were monohydric. alcoahol-modified :urea. formaldehyde-resin rand 'vegetablewoib modified, glycerol -.:phthalatei. alkyd :resin. .These resins were in solution. inv a mixtureofwbutyl alcohol and volatile-liquid hydrocarbons.

. This.=coating composition was. sprayed through conventional paint-.spraying-equipment in a -water curtain spray-booth. The atomizeduspray. dustor over-. spray was precipitated by the water and was carried to a reservoir where it accumulated as overspray sludge.

A sample of the sludge wasfound to still be soluble in an'alcohol/hydrocarbon mixture-ofthe same character asithat -contained' in the original coating composition.

Sixcontainers were-filled about 90%. .full with Waterbearing sludgeskirnmed: from the reservoir. To two conf-..tainers wasadded about 6%, by volume basedronthe ..sludge in the-.containenof-butyl alcohol. To two others was added about 6%-,-on the same .basis, of a mixturecon- =taining.20:80-by weight butyl'alcoholzvolatile liquidhy- --.drocarbou. Torthe remaining two was added about 6% by volume of water. The contents were.-not agitated, a but ailong-blade was inserted: throughihe mass .to the :bottom: of 1 the container and movedzback and forth once through the stiff sludge to'produce a temporary core. in

which the'liquid-present could collect. The containers were-closed, marked to identify the duplicates and sepwaratedinto two sets-oneset for storage at-room tem- :.perature and the'other'setfon an accelerated deteriora- .tion-test-at 120 F. Periodicallyhthecontainers were .;opened forwinspection and testingto determine the con- ..idilil10fzih6 sludge. lThe. observations are recorded in Table I. .In the table, fexcellentmeans-that thesludge was stillin substantially its=original condition; :it was soft, workable, soluble, and reclaimable 'to .yield aproduct ..having protective and decorative properties?substantially equal-to the original-coating composition. f Good means that; except'for aslight thickening-,no importantchange vxhad taken place. Fairv means that some :seed had. de-

velopedorf that considerable thickening or-lsome gelation had occurred and that. the product. :reclaimedtherefrom .-.would be expected-by previous "experience to beof inferior quality. NG means that a considerable amount of seed or-gel had formed or that the sludge had deteriorated to a .stringyasemi-solid, in either of which conditions it could' not bereclaimed to yield a product having properties substantially equal toxthe original coating composition.

Table 'STABILITYDF OVERSPRAY SLUDGE Sludge and Sludge and Butyl ButylAlcohol/ $5.3 Alcohol Hydrocarbon 'Room Temperature Storage: After 7 days Excellent.-. Excellent- Good. After 13'daysdo.-.- Good do. --.Atter 34 days do Fair NG.

1120? F. Storage: v 7

After 47 hours do 'Good '(discou- I tulued). Alter 72 hours do Fair (discontinned).

Alter 97 hours c The results of these tests show that the butyl alcohol, which is a solvent for the mixed resins in the overspray, keeps the sludge in excellent condition for at least 34 days. Although the butyl alcohol/hydrocarbon mixture, which is also a solventtor. themixed resins in the overspray, does not preserve'the overspray in such good condition, it is' considerably better than: no solvent treatment at all, as'represented bythewater covered sample.

EXAMPLE II Three-'SS-gallon drums were filled to. about? 90% of capacity with an overspray sludge having .the-same'essential constituents as that of Example I. Threegallons of butyl alcohol were added'tothe first drum,'three gallons of isopropyl alcohol to thesecond drum, and three gallons of water to the..third drum. The drums were closed and stored outdoors'in a plant'yard'for about 3 months of summer weather. Inspection after this period eta-storage showed that the sludge'in the "drums 'containinglhe butyl alcohol and isopropyl alcohol, respectively, wasis till in good condition. Both alcohols. are solventsforfilthe mixed resins in the sludge. The sludge containing'add'ed water wasin'relatively poor condition.

. Samples from each drum were separatelyreprocessed by adding enough solvent'to. thin. the oversprayltman enamel-like consistency, then by distilling ofi'substantially all of the water by azeotropic.distillationlunder vacuum, and finally, by adding solvent to adjust .the .viscosity or" the reclaimed enamel to substantiallythefsame viscosity asthe enamel from which the overspray had been collected. The resulting three enamels were tested for appearance, and a very slight seed was .visible'in'lthe two derived from thedrumsv containing butyl alcohol and isopropyl alcohol respectively. .These two products were useful, alone or blended with. new enamel ofl'substantially the same composition, as first-quality enamels. Considerable seediness was observed .inthe enamel'derived fromthe drum containing added .waterandthis product was not useful as a.first .-quality enamel.

While the examplesillustrate .the improvements'of'this invention with respect. to enamels containing urea-formaldehyde and alkyd resins, the same. process is applicable to organic. coating compositions containing. otherresin- .ous film-formers suchas oilmodifiedQalkydresins alone, blends of suchan :alkyd resin with monohydric. alcohol modified melamine formaldehyde resin, blends of..such

an alkyd resin withnitrocellulose, .and the. like,.lwhen .the overspray collected from such a coating composition deteriorates in storage to a seedy, gelled,..or..insoluble state.

different degreewith different solvents or. combinations ..thereof and, further, thatthe Choicezof aparticulansolvent for optimum results will be governed .notonly'. by its physical and chemical characteristics but also by...its availability, cost, convenience in handling, andother nontechnical aspects.

.jThe solvents .added'to preserve the sludge of. a.1given type of oversprayneed notcontain the-same solvents-.as

were used in the original coating composition,:so long'as the added-solvent 'is ofthe same character. as..the original tered in coating compositions,land is =otherwise innocous to the composition. For-instance,ketones; esters,-ethers, r and chlorinated hydrocarbons are good solventsiforunany resins used in. coating compositionsytand they -may.--be substituted, I where desirable -or necessary, -for theapreviously' mentioned alcohols and hydrocarbons.

1 Theiexamples show the==addition-of about-6.%-of@so1- vent based on the volume of the sludge. -It: ise'obvious that at'rsmaller amount of a stronger :solv'ent: can be substituted for this 6 or, conversely,- a? larger'zamounttof a weaker solvent. fit is also -obviouscthati there wouldabe no advantage inadding a larger:amountof'molvent'than 'that amount which is requiredin the completely processed reclaimed coating composition. l The mosta-ipractical and,

sludge is removed from the bulk of the precipitating medium, but this is not essential. The solvent may be added at a later time provided th overspray has not yet deteriorated and is still solubc in a solvent of the same character as the solvent portion of the coating composition from which the overspray was derived.

Although the examples employ enamels pigmented with titanium dioxide, it is obvious that pigmentation forms no part of this invention and that coating compositions containing no pigment at all or containing pigments other than titanium dioxide are within the scope of the invention. Likewise, other conventional modifying agents such as plasticizers, inhibitors, driers, and the like, form no part of this invention and they may or may not be present, depending on the nature of the coating composition.

The azeotropic distillation method of separating water from the valuable portion of the sludge, as used in Example ]1, is only one of several water-removal methods. Other methods within the scope of this invention are decantation, solvent extraction, conventional distillation, and centrifugation.

In reclaiming overspray, complete removal of the water is usually not essential because the reclaimed coating composition usually will tolerate a water content up to the order of 2%4%. Of course, if the overspray is recovered by emulsification in water, the water removal step is eliminated.

Although overspray sludge usually contains 5%-25% of water, such a concentration is not critical and merely represents the range commonly found in commercial overspray recovery processes.

From the foregoing complete description of this invention, it is seen that the process preserves the valuable constituents of excess sprayed coating composition, which has been collected as overspray sludge, in substantially their original condition for a period considerably longer than was heretofore possible. Thereby, the valuable constituents need not be reprocessed immediately in order to obtain a first-quality reclaimed product, but the reprocessing may be delayed and the deterioration of that overspray which would have occurred during the same pe riod is prevented or considerably inhibited. The net result of these advantages is that substantially the entire volume of the original coating composition can ultimately be used for its original purpose, whereas heretofore some was lost for first-quality uses by deterioration of the collected overspray, or unnecessary cost was involved in minimizing the storage period to prevent deterioration.

It is apparent that many widely different embodiments of this invention can be made without departing from the spirit and scope thereof and, therefore, it is not intended to Ibe1 limited except as defined in the appended claims.

c arm:

1. The process of reclaiming as a usable organic coating composition the collected overspray of a sprayed organic coating composition which comprises (1) placing in a closable storage vessel an aqueous sludge of the overspray of a coating composition, the organic film-forming portion of which is completely soluble in a volatile organic solvent of the same character as the volatile organic solvent portion of the coating composition from which the overspray was derived, (2) substantially preventing the formation of seed or gel therein until the sludge has been completely reprocessed by adding to the sludge at least 1%, based on the volume of the sludge, of a volatile organic solvent for the film-forming portion of the overspray, said film-forming portion being susceptible to the formation of seed or gel in the absence of said solvent, and (3) subsequently reclaiming the overspray in said sludge by separating therefrom substantially all of the Water and by dissolving the overspray in a volatile organic solvent of the same character as the volatile organic solvent portion or" the coating composition from which the overspray was derived.

2. In the process of reclaiming an organic coating composition, which process comprises (1) precipitating in an aqueous medium the overspray of a sprayed organic coating composition, said overspray in the form in which it is collected being susceptible to the formation of seed or gel therein when subjected to storage prior to the completion of said process, (2) collecting and storing an aqueous sludge containing said overspray in a closable vessel, (3) separating substantially all of the water from said sludge, and (4) dissolving said overspray in a volatile organic solvent of the same character as the volatile organic solvent portion of the coating composition from which said overspray was derived; the improvement which comprises substantially preventing the formation of seed or gel in said overspray until said process is completed by adding to the vessel containing said sludge, at the time said sludge is collected therein, at least 1%, based on the volume of the sludge, of a volatile organic solvent for the resinous film-forming portion of said overspray.

3. The process of producing a reclaimed coating composition from the overspray of a sprayed organic coating composition, the liquid organic portion of which consists essentially of a solution of monohydric alcohol-modified urea-formaldehyde resin and vegetable oil-modified alkyd resin in butyl alcohol and volatile liquid hydrocarbons which process comprises (1) placing in a closable storage vessel an aqueous sludge of said overspray, the resinous portion of which is soluble in a volatile organic solvent of the same character as the volatile organic solvent portion of the sprayed coating composition and is susceptible to the formation of seed or gel during prolonged storage, (2) adding to the sludge in said vessel a volatile organic solvent selected from the class consisting of butyl alcohol, isopropyl alcohol, liquid hydrocarbons, and mixtures thereof, in the amount of about 6% by volume of the sludge to prevent the formation of seed or gel until said reclaimed coating composition has been produced, (3) separating substantially all of the water from said sludge, and (4) dissolving said overspray in a volatile organic solvent of the same character as the volatile organic solvent portion of the sprayed coating composition, by which steps is produced a reclaimed coating composition having protective and decorative properties substantially equal to those of the sprayed coated composition.

4 The process of claim 1 in which the volatile orgame solvent added is in the amount of 1 to 10% by volume based on the volume of the sludge to which it is added.

5. The process of claim 2 in which the volatile organic solvent added is in the amount of 1 to 10% by volume based on the volume of the sludge to which it is added.

References Cited in the file of this patent UNITED STATES PATENTS 2,064,876 Bender Dec. 22, 1936 2,086,514 Saunders July 6, 1937 2,315,125 Meckler Mar. 30, 1943 

1. THE PROCESS OF RECLAIMING AS A UNSABLE ORGANIC COATING COMPOSITION THE COLLECTED OVERSPRAY OF A SPRAYED ORGANIC COATING COMPOSITION WHICH COMPRISES (1) PLACING IN A CLOSABLE STORAGE VESSEL AN AQUEOUS SLUDGE OF THE OVERSPRAY OF A COATING COMPOSITION, THE ORGANIC FILM-FORMING PORTION OF WHICH IS COMPLETELY SOLUBLE IN A VOLATILE ORGANIC SOLVENT OF THE SAME CHARACTER AS THE VOLATILE ORGANIC SOLVENT PORTION OF THE COATING COMPOSITION FROM WHICH THE OVERSPRAY WAS DERIVED, (2) SUBSTANTIALLY PREVENTING THE FORMATION OF SEED OR GEL THEREIN UNTIL THE SLUDGE HAS BEEN COMPLETELY REPROCERSSED BY ADDING TO THE SLUDGE AT LEAST 1%, BASED ON THE VOLUME OF THE SLUDGE, OF A VOLATILE ORGANIC SOLVENT FOR THE FILM-FORMING PORTION OF THE OVERSPRAY, SAID FILM-FORMING PORTION BEING SUSCEPTIBLE TO THE FORMATION OF SEED OR GEL IN THE ABSENT OF SAID SOLVENT AND (3) SUBSEQUENTLY RECLAIMING THE OVERSPRAY IN SAID SLUDGE BY SEPARATING THEREFROM SUBSTANTIALLY ALL OF THE WATER AND BY DISSOLVING THE OVERSPRAY IN A VOLATILE ORGANIC SOLVENT OF THE SAME CHARACTER AS THE VOLATILE ORGANIC SOLVENT PORTION OF THE COATING COMPOSITION FROM WHICH THE OVERSPRAY WAS DERIVED. 